The number of bicyclists killed in collisions with motor vehicles has increased 28 percent over the past decade - from 18 to 23 deaths per year, according to a Chronicle analysis of data collected by the California Highway Patrol.

That increase is despite a 22 percent drop in the number of regional bicycle accidents between 1997 and 2006 in the nine San Francisco Bay Area counties. The number of bicyclists injured in accidents over that period declined by a similar amount.

"That means more of the bicyclists who are being hit are being killed," said Sean Co, bicycling coordinator for the Metropolitan Transportation Commission.

Accidents in urban areas are most common but occur at lower speeds where injuries are more likely to be less serious. But accidents on rural roads or open highways are likely to involve higher speeds.

"Speed," Co said, "is probably the highest contributing factor in any bicycle collision that results in a fatality."

According to the CHP statistics, 179 Bay Area bicyclists have been killed and 25,715 injured in bicycle collisions with cars between 1997 and 2006. But the number of accidents and the number of injuries have each steadily decreased while the number of fatalities remained steady for years before jumping to 23 in 2006. And, based on an analysis of incomplete 2007 data, the increase in fatalities is likely to continue.